(1) Field of the Invention
The present invention relates to a pneumatic tire having a cap/base structured tread and having a remarkably improved resistance against heat build-up. A vulcanizate of a rubber composition comprising short fibers of syndiotactic (hereinafter abbreviated as syn)-1,2-polybutadiene is used as the tread base rubber of the tire.
(2) Description of the Prior Art
There have been various investigations of the internal heat build-up of tire, and it is well known that among troubles caused by the internal heat build-up, a most serious problem occurs in the lower portion of tread, that is, in the tread base portion ranging from the center portion of tread crown to the tread shoulder portion. In a large-size tire, the tread, particularly the shoulder portion, is thick, and the temperature of the shoulder portion often exceeds 100.degree. C. during the running due to the stresses and deformations repeatedly subjected to the tread. Therefore, a rubber having a low heat build-up, that is, a highly resilient rubber, is used as the tread base rubber of a cap/base structured tread, for example, as shown in FIGS. 1A, 1B and 1C. The rubber having a low heat build-up is produced at present by using a lower grade carbon black or by compounding a smaller amount of carbon black to a rubber. However, the tread base portion may be contacted with the road surface at the end of the use depending upon the structure of tire. Therefore, the tread base portion is required to have a high resistance against heat build-up and a high strength. However, a rubber composition capable of giving a vulcanizate having sufficiently high resistance against heat build-up and strength has not yet been developed. Accordingly, a pneumatic tire, which is low in the heat build-up and has a high wear resistance even at the end of the use, has not yet been obtained.